It is often the case that wells require stimulation to restart or enhance hydrocarbon flow. Such stimulation typically involves pumping stimulation fluid into the hydrocarbon bearing formation under pressure. Stimulation fluid may comprise components such as acid, sand, and energized carbon dioxide and nitrogen gases that, alone and under high pressures, can be damaging to the structural integrity and internal surfaces of a wellhead assembly that is installed at the top of a well casing or tubing. In other instances, it is preferred to localize the effects of elevated pressure in a well.
To protect a wellhead from damage including from high pressures and corrosive or erosive materials used during stimulation of a well, a wellhead isolation tool is used. Such a wellhead isolation tool typically includes a tubular mandrel inserted through the wellhead, blow out preventors (BOP) and the like and into the well tubing or casing therein, such that pressurized stimulation fluids pass through the mandrel without exposure to the wellhead and surface equipment components. To completely seal the wellhead from stimulation fluids during operation, the mandrel has a sealing means, commonly referred to as a sealing nipple or packoff nipple, at its downhole end for achieving a fluid seal against the inside of the tubing or casing while under high stimulating pressure. Such packoff nipples are very well known in the art. For example, U.S. Pat. No. 4,023,814 to Pitts, U.S. Pat. No. 4,111,261 to Oliver, 1978, Canadian Patent 1,169,766 to McLeod and U.S. Pat. No. 5,060,723 to Sutherland and Wenger disclose an annular elastomeric sealing cup attached in a fixed position to a nipple body which expands radially under high fluid pressures to form a friction seal of the annular space between the nipple body and the well tubing or casing. Oliver further discloses an elastomeric packer ring fixedly positioned above the sealing cup as a secondary sealing means. More recently, axially moveable annular elastomeric sealing members have been disclosed whereby stimulation pressures force an elastomeric member to move upwardly and extrude into a narrowing annular space, thereby resulting in an extrusion seal. For example, in U.S. Pat. No. 5,261,487 to McLeod and Roesch, a lower sealing cup expands radially and moves upwardly against an upper packer ring. The packer ring is then forced to extrude between a shoulder section projecting outwardly from the nipple body and the well casing or tubing. In U.S. Pat. No. 6,918,441 to Dallas, rather than using packer ring, a top portion of a sealing cup is extruded. In both cases, however, both a friction seal and an extrusion seal are formed. When pressure is equalized and the nipple is withdrawn from the tubing, the elastomeric members are anticipated to collapse to their original shape thereby allowing safe extraction of the wellhead isolation tool.
To remove the packoff nipple after well stimulation operations are completed, the residual well pressure is equalized above the packoff nipple in the objective of relaxing the elastomeric seals to thereby allow for safe extraction of the packoff nipple.
In general, elastomeric seals of prior art packoff nipples are susceptible to damage during well tubing or casing entry or exit, particularly when the packoff nipple must pass areas of restricted internal diameter. This can be particularly problematic with extruded seals, which typically become permanently deformed when actuated. Prior art packoff nipples are also prone to seal pre-activation during well tubing entry whereby the seals are forced from their protective running-in condition to an actuated condition, thereby increasing the likelihood of seal damage. In any case, damage to the seals does not permit the packoff nipple to be reused and also results in damaged seal material being left in the well, thereby increasing the cost of operations.
Other difficulties encountered by prior art packoff nipples include seal failure due to seal damage by exposure to extreme temperatures associated with CO2 and N2 stimulating fluids, as well as due to misalignment of the packoff nipple in the well tubing.
There is, therefore, a need for an improved packoff nipple.